Of course if you accepted external contributions to your code under a copyleft license then you can't use that part freely, but that's kind of the point.
The irony is that non-copyleft project author/maintainers want to reserve the right to exploit other people's work when they "grow the project into a business", but bellyache when Amazon "exploit" their permissively-licensed work. Freedom for me -- but not for thee.
That there is not a good standardised way to give preferential treatment to small companies does not mean it's unreasonable to want to.
Well yes. But if you:
1. Want to accept external contributions 2. Think you might want to use the entire project (which may still be mostly your own work) in proprietary code in the future.
Then a permissive license is a lot easier than the alternatives.
Especially if it's a hosted web app business?
Providing paid support a la Red Hat, paying for hardware/cloud space, or even paid binaries if you think your customers won't bother to build from source.
(Yes, one can build from source then distribute free binaries, but maybe those same customers won't even go to the trouble of finding them?)